The present invention relates to a method of molding a succession of discrete bodies on an elongated flexible element. More particularly this invention concerns the formation of a succession of insulating disks on a wire intended to serve as the inner conductor in a coaxial cable.
A coaxial cable is known having an outer solid or braided conductor of tubular shape which is normally grounded. Received coaxially within this tubular outer conductor is an inner conductor that is frequently spaced radially inwardly of the outer conductor by means of a succession of synthetic-resin insulating disks molded on the inner conductor. This air-insulated type of coaxial cable is used for example in long distance communication systems or the like.
Methods are known for making such an inner conductor. In my British Pat. No. 1,406,100 as well as my U.S. Pat. No. 3,918,864 there is disclosed a method and apparatus using a plurality of mold pairs together forming an array having a cylindrical outer surface. Each of these mold pairs has a mold cavity and an inlet passage extending between the respective cavity and a pressurized source of plastified and hardenable resin material. A valve is provided in each of these inlet passages. The wire constituting the elongated flexible element on which is to be molded a succession of discrete bodies is fed to this array of mold pairs which are sequentially closed on successive spaced-apart portions of the flexible element. The array is rotated about a central axis and as soon as each of the mold pairs has closed on the wire the respective valve is temporarily opened so as to fill this mold cavity with the synthetic resin being employed to form the insulating bodies or disks. Thereafter the valve is closed and the respective body formed in the cavity and sprue formed in the inlet passage unitary with the body harden. Thereafter the respective mold pair is opened and the conductor carrying the body and sprue is pulled from the array of mold pairs.
Such an arrangement produces an extremely high-quality product and can operate at a high rate of speed. It, however, has the considerable disadvantage that the equipment needed is very complex and quite expensive. This complexity further leads to periodic failure so that it is necessary to shut the machine down and service it frequently.
Another arrangement is known from French Pat. No. 1,006,605. In this system there is, once again, a cylindrically annular array of mold pairs. Here the device is coupled with a stationary extruder head that is pressed radially against the cylindrical outer surface of the array. The inlet passages extend radially outwardly from the mold cavities to the outer surface. Thus the array of mold pairs is rotated past the extruder head so as to sequentially align the mold-cavity inlets with the extruder head pressed against the outer surface. As each inlet is aligned with the mold head the plastified material continuously under pressure inside the head enters the mold cavity through the inlet.
This system simple as it appears to be at first sight, however, has the considerable disadvantage that there is periodically some leakage between the complementarily shaped surfaces of the nozzle and mold array, so that plastified material can escape and form lumps on the mold array. This leakage is most common at the regions between adjacent mold pairs. Furthermore the material forced under pressure into each of the mold cavities tends to push back out to a limited extent after each of these cavities passes the nozzle, thereby again frequently forming a lump on the surface of the mold array.
An attempt has been made to overcome this disadvantage by extremely finely machining the mold pairs and extruder head, and by urging the extruder head with great force against the mold array. Such a fine machining again greatly increases the cost of the installation. Furthermore the considerable radial pressure exerted by the extruder on the mold array causes considerable wear at the interface between these two elements, and greatly loads the bearing for the mold array. Thus equipment cost is again elevated and the device must be frequently shut down in order to service it and replace some part.